1. Field of the Invention
The present invention relates to a thermoforming oven for the convective heating of a continuous web of a thermoplastic foam or film material which is being advanced through the oven towards a thermoformer. Moreover, the invention pertains to a method for the improved and more uniform convective heating of a thermoplastic foam or film material which is advanced through a thermoforming oven.
In the currently utilized technology pertaining to the thermoforming of plastic film or sheet materials, especially plastic foam sheet, wherein the most different and varied types of articles are molded into the film or foam sheet in a thermoformer, various kinds of ovens are employed for imparting the required heat to the film or foam sheet material prior to the latter being conveyed to the thermoformer. Thermoforming ovens which are presently in widespread commercial use are primarily of the radiant heat type, in which a plurality of heating devices producing radiant heat are interposed along the path of travel of the thermoplastic film or foam sheet material through the thermoforming oven, so as to thereby impart the desired amount of heat to the material and to cause it to expand to a controlled extent preceding the thermoforming operation. Because of encountered fluctuations or variations in the temperatures of the radiant heater components along the expanse of the oven, and the possible sagging of the film or foam sheet while being conducted through the oven, any thermoformer installations which normally incorporate thermoforming ovens containing radiant heaters generally fail to impart an adequate degree of heating uniformity to the film or sheet material. Consequently, the film or web material can shift internally within the confines or expanse of the advancing web in response to the variations in temperature encountered across the surface of the web or sheet, thereby adversely affecting the quality of any thermoformed articles produced therefrom. In order to improve upon the heating action of such thermoforming ovens containing radiant heaters, there have been contemplated utilizations of convective hot-air ovens in order to be able to achieve a more uniform temperature distribution over the surface of the film or foam sheet in the oven. The replacement of radiant heaters with an impinging hot-air flow of a convective hot-air oven has generally not been completely successful, inasmuch as the impinging hot air usually scorches the foam sheet or film when the web is intermittently stopped in its advance through the oven during molding cycles which are being implemented in the thermoformer.
Pursuant to a more recent development, instead of employing an impinging flow of hot air for heating the film material in a convective hot air oven, there has been employed the interaction of a flow pattern of a steam and air mixture and the thermoplastic foam sheet material. Although this essentially minimizes the previously encountered scorching of the sheet of plastic film as the latter is transported through the thermoforming oven; nevertheless, the large temperature drop which is generally encountered in such convective ovens limits the article-producing speed of the thermoforming cycle. The present invention overcomes this particular limitation by providing an internal recirculation system of a heating medium designed to produce a unique countercurrent flow of such medium in a convective thermoforming oven which, to a considerable degree, reduces the temperature drop of the impinging medium flow which is in contact with the surface of the intermittently advancing foam sheet material through the expedient aspect of heat conduction between the recirculating streams of the heating medium flow.
Consequently, in comparison with conventional heat ovens employing radiant heat, the convective thermoforming oven allows for the utilization of a method in the heating by a thermoforming oven of a thermoplastic sheet of film material, which imparts a much more uniform degree of heating to an intermittently advancing thermoplastic web, and with this resulting in a more consistent weight distribution within the material as the latter is advanced from the oven into the thermoformer.
Moreover, problems relative to surface scorching encountered by intermittently advanced webs of plastic sheet or film which, at times, render conventional hot air ovens unsuitable for thermoforming processes, are largely ameliorated or even completely eliminated, inasmuch as the countercurrent flow of a heating medium, such as but not necessarily a steam-air mixture, as described hereinbelow largely overcomes the disadvantages which are presently being encountered in the technology.
2. Discussion of the Prior Art
Krutchen, et al. U.S. Pat. No. 4,485,059, assigned to the common assignee of the present application, relates to a polymer foam thermoformation process and apparatus, in which the convection oven used in implementing the thermoforming method incorporates a steam and air flow at a predetermined temperature and to certain steam to air ratios. Although this considerably improves upon the heating operations of radiant heater-type thermoforming ovens, there is no disclosure of the countercurrent flow as is contemplated by the present invention and which provides for a more uniform heating of the intermittently advanced plastic film or foam material as the latter is transported through the thermoforming oven towards a molding station.
Assarsson, et al. U.S. Pat. No. 4,279,847 relates to a method and apparatus for the continuous manufacture of foamed plastic materials wherein a stream of heated air and water is impinged against the surface of an advancing foamable plastic material. Although this eliminates the drawbacks encountered in thermoforming ovens which are equipped with radiant heaters, there is no disclosure of employing a countercurrent flow of a heating medium in a convective thermoforming oven analogous to the invention which would impart a more uniform temperature distribution to a web of thermoplastic film during an intermittent advance thereof towards a thermoformer.
Holden U.S. Pat. No. 4,438,054, also assigned to the common assignee of the present application, describes a method and apparatus for measuring and controlling foam sheet blow-up in a thermoformer oven, wherein heaters contained in the oven are controlled so as to allow for the attainment of a more uniform material thickness upon exiting the oven and prior to the conveyance thereof into a molding or forming station. This particular publication also relates to the use of radiant heaters which may cause non-uniformities in heating during an intermittent advance of the plastic film or sheet towards a thermoforming station.
Various other kinds of apparatus and methods for the heating of thermoplastic film or plastic foam sheet materials are described in Schedel U.S. Pat. No. 4,539,167; Cannon, et al. U.S. Pat. No. 4,056,587; Jacobs, et al. U.S. Pat. No. 3,189,339; Carrigan, et al. U.S. Pat. No. 3,518,334; and O'Brien, et al. U.S. Pat. No. 3,359,600. However, none of these publications disclose the use of a countercurrent flow of a steam-air mixture to attain a more uniform degree of heating over the surfacer of the sheet of an intermittently advancing thermoplastic film or web as the latter is conducted through a convective oven towards a thermoforming apparatus.